1. Field of the Invention
The present invention relates to a method for optical recording, a method for optical reproducing, an optical recording media, and an optical recording and reproducing apparatus which utilize holography, particularly digital volume holography.
2. Description of the Related Art
Optical recording media have been developed as data storage media that can record data such as high-density images with a large volume. The following optical recording media have hitherto been put to practical use: magneto-optic recording media, phase-change optical recording media, and CD-R. However, there is a growing demand for the increased volume of optical recording media. Hologram type optical recording media has been proposed which uses holography, particularly digital volume holography in order to realize large-capacity optical recording.
A method for optical recording and reproducing using holography is generally performed as described below. For recording, an information beam to which information is imparted in the form of a two-dimensional pattern is interfered with a reference beam inside an optical recording media. Information is thus recorded as interference patterns. For reproducing, recorded interference patterns are irradiated only with the reference beam. Recorded information is thus read out in the form of a two-dimensional pattern as a diffraction image from the interference patterns. Thus, this system has an advantage of making it possible to input and output information at high speed. In particular, the method for optical recording and reproducing using digital volume holography has an advantage of improving diffraction efficiency by utilizing the thickness direction of the optical recording media to three-dimensionally record the interference patterns, making it possible to record information in a specific region in the optical recording media in a multiplexed manner, and thus increasing storage capacity.
If placement (for example, the angle or position of irradiation) of the reference beam applied to the interference patterns already recorded on the optical recording media as described above is slightly shifted from its original placement for recording, the phases of the reference beam and interference patterns cannot be matched even though the recorded interference patterns are irradiated with the reference beam, thus precluding a diffraction image from being obtained. By recording interference patterns with the reference beam having the placement with which the diffraction image cannot be obtained and another information beam, it is possible to record plural pages of two-dimensional information in the same region inside the optical recording media in a multiplexed manner according to the placement of the reference beam. In this manner, the optical recording and reproducing using holography utilizes the matching between the interference patterns and the phase of light to enable multiplex recording of information. However, this implies that a diffraction beam is not obtained even if the wavelength for recording differs from that for reproducing, in other words, this method is not resistant to a wavelength variation between a recording beam and reproducing beam.
As well known, when a hologram is recorded, the use of a spatially modulated reference beam complicates recorded interference patterns, thus requiring strict phase matching conditions for the reference beam and interference patterns. Thus, the degree of recording multiplexing can be increased. For example, an optical recording and reproducing apparatus is presented which employs holography using a reference beam for recording the phase of which is spatially modulated (see Jpn. Pat. Appln. KOKAI Publication No. 2002-123949). A method has recently been disclosed which uses one spatial light modulator to generate an information beam and a modulated reference beam to record a hologram (see Hideyuki Hiromi and Kun Li, “A novel Collinear optical Setup for Holographic data Storage System”, Technical Digest of Optical Data Storage Topical Meeting 2004, pp 258-260, (2004)). However, this method requires more precise phase matching conditions for the reference beam and interference patterns. Consequently, a wavelength variation between the recording beam and reproducing beam more greatly affects recording and reproducing performance. Disadvantageously, this poses significant problems of portability of the optical recording media and compatibility between apparatuses.
A laser is commonly used to record and reconstruct a hologram. As the laser, a gas laser such as an argon laser or a helium neon laser or a solid-state laser such as a YAG laser has often been used in view of monochromaticity and coherence. However, it is preferable to use a semiconductor laser in order to provide a small-sized optical recording and reproducing apparatus with low power consumption. The semiconductor laser has already been incorporated into an optical recording and reproducing apparatus for optical recording media such as CD and DVD. The semiconductor laser enables emission of a laser beam with a sufficiently high luminance in a practical sense. However, a small variation in the composition of a material or the size or structure of elements causes problems that the semiconductor laser may oscillate at a plurality of wavelengths ant that an oscillation wavelength may vary with semiconductor lasers by several nm to about 10 nm. It is also known that the oscillation wavelength of one semiconductor laser may vary depending on temperature or the value of injection current. Therefore, it is inappropriate to use a semiconductor laser to record and reconstruct a hologram that is not resistant to the wavelength vibration. Further, to allow the same laser wavelength to be used for both recording and reproducing regardless of different environments or apparatuses, it is conceivable to incorporate a wavelength selecting structure called a DFB (Distributed FeedBack) or DBR (Distributed Bragg Reflector) into a semiconductor laser element itself and to precisely control laser manufacturing conditions so as to provide uniform characteristics. However, this method results in a low yield and high manufacturing costs. Therefore, this method is not practical.